Encapsulation of ePTFE in prevascularized collagen leads to peri-implant vascularization with reduced inflammation

Combining "waste utilization" and "tissue to tissue" strategies to accelerate vascularization for bone repair

Background

A pivotal determinant for the success of tissue regeneration lies in the establishment of sufficient vasculature. Utilizing autologous tissue grafts from donors offers the dual advantage of mitigating the risk of disease transmission and circumventing the necessity for post-transplant immunosuppression, rendering it an exemplary vascularization strategy. Among the various potential autologous donors, adipose tissue emerges as a particularly auspicious source, being both widely available and compositionally rich. Notably, adipose-derived microvascular fragments (ad-MVFs) are a promising candidate for vascularization. ad-MVFs can be isolated from adipose tissue in a short period of time and show high vascularized capacity. In this study, we extracted ad-MVFs from adipose tissue and utilized their strong angiogenic ability to accelerate bone repair by promoting vascularization.

Methods

ad-MVFs were extracted from the rat epididymis using enzymatic hydrolysis. To preserve the integrity of the blood vessels, gelatin methacryloyl (GelMA) hydrogel was chosen as the carrier for ad-MVFs in three-dimensional (3D) culture. The ad-MVFs were cultured directly on the well plates for two-dimensional (2D) culture as a control. The morphology of ad-MVFs was observed under both 2D and 3D cultures, and the release levels of vascular endothelial growth factor (VEGF) and bone morphogenetic protein 2 (BMP-2) were assessed under both culture conditions. In vitro studies investigated the impact of ad-MVFs/GelMA hydrogel on the toxicity, osteoblastic activity, and mineralization of rat bone marrow mesenchymal stem cells (rBMSCs), along with the examination of osteogenic gene and protein expression. In vivo experiments involved implanting the ad-MVFs/GelMA hydrogel into critical-size skull defects in rats, and its osteogenic ability was evaluated through radiographic and histological methods.

Results

ad-MVFs were successfully isolated from rat adipose tissue. When cultured under 2D conditions, ad-MVFs exhibited a gradual disintegration and loss of their original vascular morphology. Compared with 2D culture, ad-MVFs can not only maintain the original vascular morphology, but also connect into a network in hydrogel under 3D culture condition. Moreover, the release levels of VEGF and BMP-2 were significantly higher than those in 2D culture. Moreover, the ad-MVFs/GelMA hydrogel exhibited superior osteoinductive activity. After implanting into the skull defect of rats, the ad-MVFs/GelMA hydrogel showed obvious effects for angiogenesis and osteogenesis.

The translational potential of this article

The utilization of autologous adipose tissue as a donor presents a more direct route toward clinical translation. Anticipated future clinical applications envision the transformation of discarded adipose tissue into a valuable resource for personalized tissue repair, thereby realizing a paradigm shift in the utilization of this abundant biological material.

Isolated Fragments of Intact Microvessels: Tissue Vascularization, Modeling, and Therapeutics

The microvasculature is integral to nearly every tissue in the body, providing not only perfusion to and from the tissue, but also homing sites for immune cells, cellular niches for tissue dynamics, and cooperative interactions with other tissue elements. As a microtissue itself, the microvasculature is a composite of multiple cell types exquisitely organized into structures (individual vessel segments and extensive vessel networks) capable of considerable dynamics and plasticity. Consequently, it has been challenging to include a functional microvasculature in assembled or fabricated tissues. Isolated fragments of intact microvessels, which retain the cellular composition and structures of native microvessels, are proving effective in a variety of vascularization applications including tissue in vitro disease modeling, vascular biology, mechanistic discovery, and tissue prevascularization in regenerative therapeutics and grafting. In this review, we will discuss the importance of recapitulating native tissue biology and the successful vascularization applications of isolated microvessels.

Generation of Connective Tissue-Free Microvascular Fragment Isolates from Subcutaneous Fat Tissue of Obese Mice

BACKGROUND

Microvascular fragment (MVF) isolates are generated by short-term enzymatic digestion of adipose tissue and contain numerous vessel segments for the vascularization of tissue defects. Recent findings indicate that the functionality of these isolates is determined by the quality of the fat source. Therefore, we compared MVF isolates from subcutaneous adipose tissue of obese and lean mice.

METHODS

MVF isolates were generated from subcutaneous adipose tissue of donor mice, which received a high fat or control diet for 12 weeks. The isolates were analyzed in vitro and in vivo.

RESULTS

Feeding of mice with a high fat diet induced obesity with adipocyte hypertrophy, resulting in a significantly lower collagen fraction and microvessel density within the subcutaneous fat depots when compared to lean controls. Accordingly, MVF isolates from obese mice also contained a reduced number of MVF per mL adipose tissue. However, these MVF tended to be longer and, in contrast to MVF from lean mice, were not contaminated with collagen fibers. Hence, they could be freely seeded onto collagen-glycosaminoglycan scaffolds, whereas MVF from lean controls were trapped in between large amounts of collagen fibers that clogged the pores of the scaffolds. In line with these results, scaffolds seeded with MVF isolates from obese mice exhibited a significantly improved in vivo vascularization after implantation into full-thickness skin defects.

CONCLUSION

Subcutaneous adipose tissue from obese mice facilitates the generation of connective tissue-free MVF isolates. Translated to clinical conditions, these findings suggest that particularly obese patients may benefit from MVF-based vascularization strategies.

18-Fluorodeoxyglucose positron emission tomography in the diagnosis of prosthetic aortic graft infection: the difference between open and endovascular repair

OBJECTIVES

18-Fluorodeoxyglucose (18F-FDG) positron emission tomography (PET)/computed tomography (CT) has been reported as useful for diagnosing aortic graft infection. However, 18F-FDG uptake may depend upon various factors including open versus endovascular repair and time from surgery. We aimed to elucidate the factors influencing its uptake and the diagnostic value of 18F-FDG PET/CT after open and endovascular repair.

METHODS

Hospital database of PET/CT (N = 14 490) and our departmental database were cross-checked to identify those who underwent 18F-FDG PET/CT after aortic repair. Patient's data were retrieved from the chart. Images were reviewed by 2 nuclear medicine specialists in consensus, and the presence of increased 18F-FDG uptake was recorded. The maximum standardized uptake value (SUV max) was measured.

RESULTS

Among the 1112 patients who underwent aortic repair between 2011 and 2022, 71 patients were identified. Eighteen patients underwent 18F-FDG PET/CT for suspected graft infection and the remaining 53 patients for other purposes (malignancy, etc.). Fourteen patients were treated as aortic graft infection. They had significantly higher SUV max than those without graft infection [mean 8.64 (standard deviation 2.78) vs 3.40 (standard deviation 0.84); P < 0.01]. In the non-infected grafts, SUV max was higher early after open surgical repair, while it remained low after endovascular repair.

CONCLUSIONS

After endovascular aortic repair, a constant cut-off value of 'SUV max = 4.5' seems appropriate for diagnosing graft infection, since it remains low and stable from the early postoperative period. After open surgical repair, it seems acceptable to have 'stepwise cut-off value' depending on the time from surgery.

Methods for vascularization and perfusion of tissue organoids

Tissue organoids or "mini organs" can be invaluable tools for understanding health and disease biology, modeling tissue dynamics, or screening potential drug candidates. Effective vascularization of these models is critical for truly representing the in vivo tissue environment. Not only is the formation of a vascular network, and ultimately a microcirculation, essential for proper distribution and exchange of oxygen and nutrients throughout larger organoids, but vascular cells dynamically communicate with other cells to modulate overall tissue behavior. Additionally, interstitial fluid flow, mediated by a perfused microvasculature, can have profound influences on tissue biology. Thus, a truly functionally and biologically relevant organoid requires a vasculature. Here, we review existing strategies for fabricating and incorporating vascular elements and perfusion within tissue organoids.

The Evaluation of Neovessel Angiogenesis Behavior at Tissue Interfaces

Angiogenesis, the formation of new vessel elements from existing vessels, is important in homeostasis and tissue repair. Dysfunctional angiogenesis can contribute to numerous pathologies, including cancer, ischemia, and chronic wounds. In many instances, growing vessels must navigate along or across tissue-associated boundaries and interfaces tissue interfaces. To understand this dynamic, we developed a new model for studying angiogenesis at tissue interfaces utilizing intact microvessel fragments isolated from adipose tissue. Isolated microvessels retain their native structural and cellular complexity. When embedded in a 3D matrix, microvessels, sprout, grow, and connect to form a neovasculature. Here, we discuss and describe methodology for one application of our microvessel-based angiogenesis model, studying neovessel behavior at tissue interfaces.

Adipose Tissue-Derived Microvascular Fragments From Male and Female Fat Donors Exhibit a Comparable Vascularization Capacity

Adipose tissue-derived microvascular fragments (MVF) represent effective vascularization units for tissue engineering. Most experimental studies exclusively use epididymal fat tissue of male donor mice as a source for MVF isolation. However, in future clinical practice, MVF-based approaches may be applied in both male and female patients. Therefore, we herein compared the vascularization capacity of MVF isolated from the epididymal and peri-ovarian fat tissue of male and female donor mice. Freshly isolated MVF from male and female donors did not differ in their number, length distribution, viability and cellular composition. After their assembly into spheroids, they also exhibited a comparable in vitro sprouting activity. Moreover, they could be seeded onto collagen-glycosaminoglycan matrices, which were implanted into full-thickness skin defects within mouse dorsal skinfold chambers. Repetitive intravital fluorescence microscopy as well as histological and immunohistochemical analyses revealed a comparable vascularization and incorporation of implants seeded with MVF of male and female origin. Taken together, these findings demonstrate that the vascularization capacity of MVF is not gender-specific.

Microvascular fragments in microcirculation research and regenerative medicine

Adipose tissue-derived microvascular fragments (MVF) are functional vessel segments, which rapidly reassemble into new microvasculatures under experimental in vitro and in vivo conditions. Accordingly, they have been used for many years in microcirculation research to study basic mechanisms of endothelial cell function, angiogenesis and microvascular network formation in two- and three-dimensional environments. Moreover, they serve as vascularization units for musculoskeletal regeneration and implanted biomaterials as well as for the treatment of myocardial infarction and the generation of prevascularized tissue organoids. Besides, multiple factors determining the vascularization capacity of MVF have been identified, including their tissue origin and cellular composition, the conditions for their short- and long-term storage as well as their implantation site and the general health status and medication of the recipient. The next challenging step is now the successful translation of all these promising experimental findings into clinical practice. If this succeeds, a multitude of future therapeutic applications may significantly benefit from the remarkable properties of MVF.

Vascularization of Microvascular Fragment Isolates from Visceral and Subcutaneous Adipose Tissue of Mice

Background:

Adipose tissue-derived microvascular fragments (MVF) represent effective vascularization units for tissue engineering. Most experimental studies in rodents exclusively use epididymal adipose tissue as a visceral fat source for MVF isolation. However, in future clinical practice, MVF may be rather isolated from liposuctioned subcutaneous fat tissue of patients. Therefore, we herein compared the vascularization characteristics of MVF isolates from visceral and subcutaneous fat tissue of murine origin.

Methods:

MVF isolates were generated from visceral and subcutaneous fat tissue of donor mice using two different enzymatic procedures. For in vivo analyses, the MVF isolates were seeded onto collagen-glycosaminoglycan scaffolds and implanted into full-thickness skin defects within dorsal skinfold chambers of recipient mice.

Results:

By means of the two isolation procedures, we isolated a higher number of MVF from visceral fat tissue when compared to subcutaneous fat tissue, while their length distribution, viability and cellular composition were comparable in both groups. Intravital fluorescence microscopy as well as histological and immunohistochemical analyses revealed a significantly reduced vascularization of implanted scaffolds seeded with subcutaneous MVF isolates when compared to implants seeded with visceral MVF isolates. Light and scanning electron microscopy showed that this was due to high amounts of undigested connective tissue within the subcutaneous MVF isolates, which clogged the scaffold pores and prevented the interconnection of individual MVF into new microvascular networks.

Conclusion:

These findings indicate the need for improved protocols to generate connective tissue-free MVF isolates from subcutaneous fat tissue for future translational studies.

Vascularized adipocyte organoid model using isolated human microvessel fragments

Tissue organoids are proving valuable for modeling tissue health and disease in a variety of applications. This is due, in part, to the dynamic cell-cell interactions fostered within the 3D tissue-like space. To this end, the more that organoids recapitulate the different cell-cell interactions found in native tissue, such as that between parenchyma and the microvasculature, the better the fidelity of the model. The microvasculature, which is comprised of a spectrum of cell types, provides not only perfusion in its support of tissue health, but also important cellular interactions and biochemical dynamics important in tissue phenotype and function. Here, we incorporate whole, intact human microvessel fragments isolated from adipose tissue into organoids to form both MSC and adipocyte vascularized organoids. Isolated microvessels retain their native structure and cell composition, providing a more complete representation of the microvasculature within the organoids. Microvessels expanded via sprouting angiogenesis within organoids comprised of either MSCs or MSC-derived adipocytes and grew out of the organoids when placed in a 3D collagen matrix. In MSC organoids, a ratio of 50 MSCs to 1 microvessel fragment created the optimal vascularization response. We developed a new differentiation protocol that enabled the differentiation of MSCs into adipocytes while simultaneously promoting microvessel angiogenesis. The adipocyte organoids contained vascular networks, were responsive in a lipolysis assay, and expressed the functional adipocyte markers adiponectin and PPARγ. The presence of microvessels promoted insulin receptor expression by adipocytes and modified IL-6 secretion following a TNF-alpha challenge. Overall, we demonstrate a robust method for vascularizing high cell-density organoids with potential implications for other tissues as well.

Stromal Cells Promote Neovascular Invasion Across Tissue Interfaces

Vascular connectivity between adjacent vessel beds within and between tissue compartments is essential to any successful neovascularization process. To establish new connections, growing neovessels must locate other vascular elements during angiogenesis, often crossing matrix and other tissue-associated boundaries and interfaces. How growing neovessels traverse any tissue interface, whether part of the native tissue structure or secondary to a regenerative procedure (e.g., an implant), is not known. In this study, we developed an experimental model of angiogenesis wherein growing neovessels must interact with a 3D interstitial collagen matrix interface that separates two distinct tissue compartments. Using this model, we determined that matrix interfaces act as a barrier to neovessel growth, deflecting growing neovessels parallel to the interface. Computational modeling of the neovessel/matrix biomechanical interactions at the interface demonstrated that differences in collagen fibril density near and at the interface are the likely mechanism of deflection, while fibril alignment guides deflected neovessels along the interface. Interestingly, stromal cells facilitated neovessel interface crossing during angiogenesis via a vascular endothelial growth factor (VEGF)-A dependent process. However, ubiquitous addition of VEGF-A in the absence of stromal cells did not promote interface invasion. Therefore, our findings demonstrate that vascularization of a tissue via angiogenesis involves stromal cells providing positional cues to the growing neovasculature and provides insight into how a microvasculature is organized within a tissue.

3D Bioprinting the Cardiac Purkinje System Using Human Adipogenic Mesenchymal Stem Cell Derived Purkinje Cells

PURPOSE

The objective of this study was to reprogram human adipogenic mesenchymal stem cells (hADMSCs) to form Purkinje cells and to use the reprogrammed Purkinje cells to bioprint Purkinje networks.

METHODS

hADMSCs were reprogrammed to form Purkinje cells using a multi-step process using transcription factors ETS2 and MESP1 to first form cardiac progenitor stem cells followed by SHOX2 and TBX3 to form Purkinje cells. A novel bioprinting method was developed based on Pluronic acid as the sacrificial material and type I collagen as the structural material. The reprogrammed Purkinje cells were used in conjunction with the novel bioprinting method to bioprint Purkinje networks. Printed constructs were evaluated for retention of functional protein connexin 40 (Cx40) and ability to undergo membrane potential changes in response to physiologic stimulus.

RESULTS

hADMSCs were successfully reprogrammed to form Purkinje cells based on the expression pattern of IRX3, IRX5, SEMA and SCN10. Reprogrammed purkinje cells were incorporated into a collagen type-1 bioink and the left ventricular Purkinje network was printed using anatomical images of the bovine Purkinje system as reference. Optimization studies demonstrated that 1.8 mg/mL type-I collagen at a seeding density of 300,000 cells per 200 µL resulted in the most functional bioprinted Purkinje networks. Furthermore, bioprinted Purkinje networks formed continuous syncytium, retained expression of vital functional gap junction protein Cx40 post-print, and exhibited membrane potential changes in response to electric stimulation and acetylcholine evaluated by DiBAC₄(5), an electrically responsive dye.

CONCLUSION

Based on the results of this study, hADMSCs were successfully reprogrammed to form Purkinje cells and bioprinted to form Purkinje networks.

Microvascular Fragments: More Than Just Natural Vascularization Units

Adipose tissue-derived microvascular fragments serve as natural vascularization units in angiogenesis research and tissue engineering due to their ability to rapidly reassemble into microvascular networks. Recent studies indicate that they exhibit additional unique properties that may be beneficial for a wide range of future biomedical applications. Their angiogenic activity can be increased during short-term cultivation as a means of adapting their vascularization capacity to patient-specific needs. Moreover, they are a source of endothelial progenitor cells, multipotent mesenchymal stromal cells, and lymphatic vessel fragments. Finally, they exert immunomodulatory effects, determining the tissue integration of implanted biomaterials. Hence, microvascular fragments represent versatile building blocks for the improvement of vascularization, organotypic tissue formation, lymphatic regeneration, and implant integration.

Degradation, Bone Regeneration and Tissue Response of an Innovative Volume Stable Magnesium-Supported GBR/GTR Barrier Membrane

Introduction: Bioresorbable collagenous barrier membranes are used to prevent premature soft tissue ingrowth and to allow bone regeneration. For volume stable indications, only non-absorbable synthetic materials are available. This study investigates a new bioresorbable hydrofluoric acid (HF)-treated magnesium (Mg) mesh in a native collagen membrane for volume stable situations. Materials and Methods: HF-treated and untreated Mg were compared in direct and indirect cytocompatibility assays. In vivo, 18 New Zealand White Rabbits received each four 8 mm calvarial defects and were divided into four groups: (a) HF-treated Mg mesh/collagen membrane, (b) untreated Mg mesh/collagen membrane (c) collagen membrane and (d) sham operation. After 6, 12 and 18 weeks, Mg degradation and bone regeneration was measured using radiological and histological methods. Results: In vitro, HF-treated Mg showed higher cytocompatibility. Histopathologically, HF-Mg prevented gas cavities and was degraded by mononuclear cells via phagocytosis up to 12 weeks. Untreated Mg showed partially significant more gas cavities and a fibrous tissue reaction. Bone regeneration was not significantly different between all groups. Discussion and Conclusions: HF-Mg meshes embedded in native collagen membranes represent a volume stable and biocompatible alternative to the non-absorbable synthetic materials. HF-Mg shows less corrosion and is degraded by phagocytosis. However, the application of membranes did not result in higher bone regeneration.

Autologous Vascularization: A Method to Enhance the Antibacterial Adhesion Properties of ePTFE

BACKGROUND

Expanded polytetrafluoroethylene (ePTFE), an ideal bioimplant material, is commonly used in surgical repair to treat soft tissue defects and deformities. However, the main disadvantage of ePTFE is that its distinctive porous ultrastructure is prone to bacterial adhesion that gives rise to infection and chronic inflammation, resulting in functional failure. Herein, a potentially promising approach to ePTFE autologous vascularization (AV-ePTFE) in vivo was established and developed to enhance the material's antibacterial properties.

METHODS

Hematoxylin and eosin (H&E) staining and visual observation were performed to validate the intensity of the inflammatory response and related histological changes in surgical wounds after AV-ePTFE implantation. In addition, the antibacterial activities of AV-ePTFE were assessed by an in vitro bacterial adhesion assay and scanning electron microscope observation.

RESULTS

The optimal time point of AV-ePTFE was 12 weeks after implantation. AV-ePTFE relieved inflammation based on an inflammation grading evaluation and expedited wound healing. Furthermore, AV-ePTFE effectively reduced the number of bacterial adhesions, inhibited bacterial biofilm formation, and prevented the occurrence of infection.

CONCLUSIONS

We conclude that autologous vascularization is an effective method to improve the antibacterial adhesion properties and biocompatibility of ePTFE after implantation and that it may have a significant effect on clinical application of future porous biomaterials.

Prevascularized silicon membranes for the enhancement of transport to implanted medical devices

Recent advances in drug delivery and sensing devices for in situ applications are limited by the diffusion-limiting foreign body response of fibrous encapsulation. In this study, we fabricated prevascularized synthetic device ports to help mitigate this limitation. Membranes with rectilinear arrays of square pores with widths ranging from 40 to 200 μm were created using materials (50 μm thick double-sided polished silicon) and processes (photolithography and directed reactive ion etching) common in the manufacturing of microfabricated sensors. Vascular endothelial cells responded to membrane geometry by either forming vascular tubes that extended through the pore or completely filling membrane pores after 4 days in culture. Although tube formation began to predominate overgrowth around 75 μm and continued to increase at even larger pore sizes, tubes formed at these large pore sizes were not completely round and had relatively thin walls. Thus, the optimum range of pore size for prevascularization of these membranes was estimated to be 75-100 μm. This study lays the foundation for creating a prevascularized port that can be used to reduce fibrous encapsulation and thus enhance diffusion to implanted medical devices and sensors. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1602-1609, 2016.

Implantable tissue isolation chambers for analyzing tumor dynamics in vivo

Recruitment of new blood vessels from the surrounding tissue is central to tumor progression and involves a fundamental transition of the normal, organized vasculature into a dense disarray of vessels that infiltrates the tumor. At present, studying the co-development of the tumor and recruited normal tissue is experimentally challenging because many of the important events occur rapidly and over short length scales in a dense three-dimensional space. To overcome these experimental limitations, we partially confined tumors within biocompatible and optically clear tissue isolation chambers (TICs) and implanted them in mice to create a system that is more amenable to microscopic analysis. Our goal was to integrate the tumor into a recruited host tissue - complete with vasculature - and demonstrate that the system recapitulates relevant features of the tumor microenvironment. We show that the TICs allow clear visualization of the cellular events associated with tumor growth and progression at the host-tumor interface including cell infiltration, matrix remodeling and angiogenesis. The tissue within the chamber is viable for more than a month, and the process is robust in both the skin and brain. Treatment with losartan, an angiotensin II receptor antagonist, decreased the collagen density and fiber length in the TIC, consistent with the known activity of this drug. We further show that collagen fibers display characteristic tumor signatures and play a central role in angiogenesis, guiding the migration of tethered endothelial sprouts. The methodology combines accessible methods of microfabrication with animal models and will enable more informative studies of the cellular mechanisms of tumor progression.

Prevascularization in tissue engineering: Current concepts and future directions

The survival of engineered tissue constructs during the initial phase after their implantation depends on the rapid development of an adequate vascularization. This, in turn, is a major prerequisite for the constructs' long-term function. 'Prevascularization' has emerged as a promising concept in tissue engineering, aiming at the generation of a preformed microvasculature in tissue constructs prior to their implantation. This should shorten the time period during which the constructs are avascular and suffer hypoxic conditions. Herein, we provide an overview of current strategies for the generation of preformed microvascular networks within tissue constructs. In vitro approaches use cell seeding, spheroid formation or cell sheet technologies. In situ approaches use the body as a natural bioreactor to induce vascularization by angiogenic ingrowth or flap and arteriovenous (AV)-loop techniques. In future, these strategies may be supplemented by the transplantation of adipose tissue-derived microvascular fragments or the in vitro generation of highly organized microvascular networks by means of sophisticated microscale technologies and microfluidic systems. The further advancement of these prevascularization concepts and their adaptation to individual therapeutic interventions will markedly contribute to a broad implementation of tissue engineering applications into clinical practice.

Biomedical implant capsule formation: lessons learned and the road ahead

Clinicians and investigators have been implanting biomedical devices into patients and experimental animals for centuries. There is a characteristic complex inflammatory response to the presence of the biomedical device with diverse cell signaling, followed by migration of fibroblasts to the implant surface and the eventual walling off of the implant in a collagen capsule. If the device is to interact with the surrounding tissues, the collagen envelope will eventually incapacitate the device or myofibroblasts can cause capsular contracture with resulting distortion, migration, or firmness. This review analyzes the various tactics used in the past to modify or control capsule formation with suggestions for future investigative approaches.

Vascularization and cellular isolation potential of a novel electrospun cell delivery vehicle

A clinical need exists for a cell delivery device that supports long-term cell viability, cell retention within the device and retrieval of delivered cells if necessary. Previously, cell isolation devices have been based on hollow fiber membranes, porous polymer scaffolds, alginate systems, or micro-machined membranes. We present the development and characterization of a novel dual porosity electrospun membrane based device, which supports cellular infiltration and vascularization of its outer porous layer and maintains cellular isolation within a lumen bounded by an inner low porosity layer. Electrospinning conditions were initially established to support electrospun fiber deposition onto nonconductive silicone surfaces. With these parameters established, devices for in vivo evaluations were produced using nylon as a nonconductive scaffold for deposition of dual porosity electrospun fibers. The outer porous layer supported the development of a penetrating microcirculation and the membrane supported the transfer of insulin from encapsulated sustained release pellets for 4 weeks. Viable cells implanted within the device could be identified after 2 weeks of implantation. Through the successful demonstration of survival and cellular isolation of human epithelial cells within the implanted devices and the ability to use the device to deliver insulin, we have established the utility of this device toward localized cell transplantation. The cell delivery device establishes a platform to test the feasibility of approaches to cell dose control and cell localization at the site of implantation in the clinical use of modified autologous or allogeneic cells.

Determinants of microvascular network topologies in implanted neovasculatures

OBJECTIVE

During neovascularization, the end result is a new functional microcirculation composed of a network of mature microvessels with specific topologies. Although much is known concerning the mechanisms underlying the initiation of angiogenesis, it remains unclear how the final architecture of microcirculatory beds is regulated. To begin to address this, we determined the impact of angiogenic neovessel prepatterning on the final microvascular network topology using a model of implant neovascularization.

METHODS AND RESULTS

We used 3D direct-write bioprinting or physical constraints in a manner permitting postangiogenesis vascular remodeling and adaptation to pattern angiogenic microvascular precursors (neovessels formed from isolated microvessel segments) in 3D collagen gels before implantation and subsequent network formation. Neovasculatures prepatterned into parallel arrays formed functional networks after 4 weeks postimplantation but lost the prepatterned architecture. However, maintenance of uniaxial physical constraints during postangiogenesis remodeling of the implanted neovasculatures produced networks with aligned microvessels, as well as an altered proportional distribution of arterioles, capillaries, and venules.

CONCLUSIONS

Here we show that network topology resulting from implanted microvessel precursors is independent from prepatterning of precursors but can be influenced by a patterning stimulus involving tissue deformation during postangiogenesis remodeling and maturation.